The prevent invention relates to a process and to a system for controlling the mixture composition for a spark ignition engine with an NOx storage catalyst during a regeneration phase.
The pollutant emission of spark ignition engines can be effectively reduced by a catalytic aftertreatment. This essentially involves the removal of harmful constituents from the exhaust gas. A catalyst promotes the afterburning of reactive CO and HC to form harmless carbon dioxide (CO2) and water (H2O) and simultaneously reduces nitrogen oxides (NOx) occurring in the exhaust gas to neutral nitrogen (N2).
A three-way catalyst, for example, is customary which simultaneously reduces all three pollutants—CO, HC and NOx. It has a tube structure made of a ceramic material which is coated with precious metals, preferably with platinum and rhodium, which accelerate the chemical reduction of the pollutants.
The catalytic three-way process requires that the mixture has a stoichiometric composition. A stoichiometric mixture composition is characterized by an air ratio λ=1.00. In the case of this mixture composition, the catalyst operates at a very high efficiency. Even a deviation of only one percent considerably impairs the efficiency of the pollutant conversion.
To regulate the mixture, the known λ-probe supplies a signal related to the momentary mixture composition to the control unit. The λ-probe is installed in the exhaust pipe of the engine at a point at which the exhaust gas homogeneity required for the functioning of the system exists over the entire operating range of the engine.
For reasons of consumption, it is desirable to operate spark ignition Otto engines in a manner similar to the operation of diesel engines with an excess of air (i.e., lean with λ>1), in as many operating conditions as possible in order to reduce the throttling losses during the charge cycle. The achievable λ-values are a function of the mixture preparation method of the basic engine and, in the case of stratified charge engines or direct injection engines, may reach a six-fold air excess (λ=6).
However, in this lean operating mode, the known three-way catalysts are useless because they require a stoichiometric (λ=1) mixture and exhaust gas in order to convert the nitrogen oxides (NOx).
To solve this problem, NOx storage catalysts are used which, during the lean operation, remove NOx from the exhaust gas and store it. Regeneration phases are artificially generated by the engine timing gear when, for example, the NOx content of the exhaust gas behind the NOx storage catalyst exceeds a predetermined threshold value. The regeneration is normally started by an adjustment of the λ-value of the main combustion (λM) from a lean value to a rich value of less than 1, for example, 0.76. It will be terminated when a predetermined time period of the rich phase has ended. At that moment, the mixture is adjusted back to lean.
There is a large number of publications on the problem of NOx storage and the regeneration of the NOx storage catalyst. With respect to engine timing, the objects are two-fold: First, the load condition of catalyst should be detected, and second, in the NOx regeneration phase, the reducing agents should be provided precisely in accordance with the requirement because the reducing agents are also composed of test-relevant pollutants. Furthermore, the change-over operation between the lean and rich phases should not be noticeable to the driver. Without exception, the known processes operate with time-controlled strategies of a rich-lean change-over whose pulse duty ratio is determined in a more or less costly manner by the detection of the occurring NOx amount. Strategies are also known, which measure the breaking-through of the storage device using an NOx sensor and, as required, trigger a regeneration. Concerning the NOx sensors, it is also known that they have a considerable transverse sensitivity to NH3, so that they can indicate the correct NOx concentration only in the case of a stoichiometric and overstoichiometric exhaust gas composition (λ≧1; NH3-free).
The known processes require considerable coordination expenditures, particularly when taking into account different load conditions. The aging behavior and the degree of sulfur poisoning of the NOx storage catalyst can be controlled only at considerable control-related expenditures and definitely not in a reliable manner. In other words, the processing speed changes during the regeneration process because it is a function of the aging and sulfur poisoning condition. The fixed duration and λM during regeneration, in the case of aged or poisoned NOx storage catalysts, inevitably result in a significant increase of the CO and HC emissions.
FIG. 5 is a schematic representation of a known directly injecting Otto engine with an NOx storage catalyst.
In FIG. 5, reference number 1 indicates an internal-combustion engine with four cylinders A to D; reference number 2 indicates an air filter; reference number 3 indicates a throttle valve; reference number 20 indicates intake pipes; and reference number 40 indicates swirl flaps for generating the turbulence for the respective cylinders A to D; reference number 9 indicates an exhaust pipe; reference numbers 4a and 4b indicate a respective three-way precatalyst for the cylinder groups B, C and A, D, respectively; reference number 5 indicates an NOx storage catalyst; reference number 10a indicates a λM sensor; reference number 10b indicates a λK/NOx sensor; and reference number 8 indicates a rear muffler.
FIG. 6 is a time lapse diagram of the λ-values λM in front of the NOx storage catalyst and λK behind the NOx storage catalyst in the case of a regeneration phase of the NOx storage catalyst of the Otto engine according to FIG. 5.
As illustrated in FIG. 6, the λM value is changed at a defined time abruptly from a lean value of 1.45 to a rich value of 0.76. On the whole, the regeneration phase according to FIG. 6 takes five seconds, after which the λM is set back to the original lean value of 1.45.
With respect to the λK value behind the NOx storage catalyst, three different phases can be recognized. In phase 1, a complete conversion takes place of the offered reducing agent (CO, HC). The value of λK is therefore barely above 1. In phase 2, a decelerated conversion of the offered reducing agent takes place and therefore a breaking-though of HC and CO. In this phase 2, the value of λK falls slightly below the stoichiometric value, specifically in the illustrated example, to approximately 0.92. In phase 3, an attenuation of the conversion of the offered reducing agent takes place in connection with a drop of the λK value to the λM value. Correspondingly, the harmful exhaust HC/CO breaking-through takes place in this phase.
The present invention is based on the recognition that a λK curve would be desirable which is indicated in FIG. 6 by the broken line S2, and extends in an approximately constant manner at λK approximately equal to 1 to the end of the regeneration phase. A corresponding course of the λM value is indicated in FIG. 6 by a dash-dotted line S1. In other words, it would have to be provided that the λM value in front of the NOx storage catalyst is raised with the start of the decelerated conversion of the offered reducing agent in order to counteract the breaking-through of the HC/CO and the resulting lowering of the λK value.
It is therefore an object of the present invention to provide a process and a system for controlling the mixture composition for an Otto engine with an NOx storage catalyst during a regeneration phase which, when the NOx storage catalysts are aged or poisoned, result in no increased CO and HC emissions.
The process according to the invention for controlling the mixture composition for an Otto engine with an NOx storage catalyst during a regeneration phase and the corresponding system respectively have the advantage that a system designed or operated according to the invention automatically adapts itself to the characteristics of different catalyst coatings or different aging or sulfur poisoning conditions. In addition, from the control behavior, the condition of the storage catalyst can be determined regarding the sulfur regeneration requirement or for diagnostic purposes.
In the present invention, the λM value of the exhaust gas fed to the NOx storage catalyst in the course of the regeneration phase is controlled with a closed loop control circuit to keep the λK value of the exhaust gas emitted from the NOx storage catalyst. Preferably, the control starts only from a predetermined intervention value λi.
In the invention, a commercially available NOx sensor is preferably used downstream of the NOx storage catalyst, which sensor measures the λK of the exhaust gas, in addition to the NOx concentration. The λK behind the NOx storage catalyst, when enriched, remains stoichiometric (=1) or overstoichiometric (>1) precisely, independently of the λM in front of the NOx storage catalyst, as long as the NOx storage catalyst can completely process the offered reducing agent (CO, HC).
According to a preferred further development, the control is set up such that the λK value of the exhaust gas emitted by the NOx storage catalyst is maintained at about one, i.e., it does not fall below a value of one or falls only insignificantly below a value of one. As a result, the catalyst operates at high efficiency. Preferably, the λK value is maintained between 0.98 and 1.02, more preferably between 0.99 and 1.01, most preferably between 0.995 and 1.005. The controlled range of λK varies depending on the design criteria of a specific application.
According to another preferred further development, the regeneration phase is initiated when the NOx content of the exhaust gas emitted by the NOx storage catalyst exceeds a predetermined value.
According to another preferred further development, the regeneration phase will be terminated when the signal of an NOx sensor behind the NOx storage catalyst and/or its rise exceeds a predetermined value. Advantageously, the NH3 transverse sensitivity of an NOx sensor downstream of the NOx storage catalyst can be utilized which signals the conclusion of the storage device regeneration considerably sooner than the λK signal. It is useful to detect the end of the regeneration not only by way of the threshold value of the NOx signal (NH3 signal) but also its rise.
According to another preferred further development, the regeneration phase is terminated when the λK value of the exhaust gas emitted by the NOx storage catalyst falls below a predetermined value. However, because of a long gas transit time between the combustion space (reducing agent production) and the measuring site (downstream of the NOx storage catalyst), this approach is less effective.
According to another preferred further development, the λM value of the exhaust gas fed to the NOx storage catalyst is controlled at the beginning of the regeneration phase to a constant pilot value λREG and the control of λK to one is not started before the λK value of the exhaust gas emitted by the NOx storage catalyst falls below a predetermined intervention value λi. It was found to be favorable to use the signal of the λ-probe close to the engine together with a pilot value 0.76<λREG<1 adapted to the regeneration task for controlling the mixture composition in order to ensure the required control speed and accuracy. In the further course of the regeneration, the λM value is modified corresponding to the progress of the regeneration by a predetermined fixed or adaptive algorithm.
According to another preferred further development, the constant pilot value λREG is obtained by making a predetermined afterinjection and controlling the λ-value of the main combustion such that the λM value of the exhaust gas fed to the NOx storage catalyst assumes the constant pilot value λREF.
The required reducing agent can be provided by overenriching the combustion (main combustion) utilized for generating the power of the internal-combustion engine, as well as by an afterinjection. When the reducing agent is added by an overenriched main combustion (0.76<λM<1.0), CO is preferably generated. When the reducing agent is added by an afterinjection, HC is produced as the reducing agent. When CO is preferably used as a reducing agent, in phase 1 of the regeneration, an undesirable breaking-through of NOx takes place which predominantly comprises of NO (nitrogen monoxide).
In the case of a preferred use of HC in this phase 1, this breaking-through of NOx does not take place or is at least considerably reduced. The cause is the competitive situation which exists during phase 1 between the NOx storage device and O2 storage devices of the catalysts. Before the O2 storage devices are evacuated, CO can not reliably reduce NOx to N2. This embodiment therefore suggests that, during phase 1 of the regeneration, the HC content of the exhaust gas be adjusted by an afterinjection to, for example, a value>3,000 ppm in front of the NOx storage catalyst, and the phases 2 and 3 be regenerated by enriching the main combustion and, in the process, control the λ value of the main combustion. This is advantageous because high HC concentrations are no longer converted as the regeneration progresses and result in the breaking-through of HC.
According to another preferred further development, the control device controls the λ-value of the exhaust gas fed to the NOx storage catalyst according to the following relation:λM=λREG+k*(λi−λK) wherein k is a constant; λM is the λ-value of the main combustion; and λK is the λ-value of the exhaust gas emitted by the NOx storage catalyst.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.